Improved ceramic compositions for high stability capacitors

ABSTRACT

A ceramic composition useful in forming capacitors and having the following composition expressed in mole percent of the metal oxides present: NEODYMIUM OXIDE - ABOUT 12 TO ABOUT 20 MOLE PERCENT; BARIUM OXIDE - ABOUT 12 TO ABOUT 20 MOLE PERCENT; TITANIUM OXIDE - ABOUT 60 TO ABOUT 70 MOLE PERCENT; BISMUTH OXIDE - ABOUT 1.5 TO ABOUT 5 MOLE PERCENT; ZIRCONIUM OXIDE - 0 TO ABOUT 5 MOLE PERCENT; STANNIC OXIDE - 0 TO ABOUT 5 MOLE PERCENT; CALCIUM OXIDE AND/OR STRONTIUM OXIDE - 0 TO ABOUT 10 MOLE PERCENT RARE EARTH OXIDES OTHER THAN NEODYMIUM OXIDE - RANGING UP TO ABOUT ONE MOLE PERCENT BUT PREFERABLY LESS THAN ABOUT 0.6 MOLE PERCENT; TITANIUM OXIDE + STANNIC OXIDE + ZIRCONIUM OXIDE - ABOUT 60 TO 70 MOLE PERCENT, AND BARIUM OXIDE + CALCIUM OXIDE + STRONTIUM OXIDE - ABOUT 12 TO ABOUT 25 MOLE PERCENT. A dense, substantially non-porous, ceramic dielectric material fired at a suitable temperature within the range of about 1220*C to about 1300*C., having a K value of about 55 to about 90, a temperature coefficient of dielectric constant (TC) of about -100 to about +100 X 10 6/*C, a % D.F. less than 0.1, and an insulation resistance corresponding to values of at least about 200 ohm-farads ranging up to about 40,000 or more ohm-farads at 125*C.

United States Patent 1191 Roup r NOV. 27, 1973 IMPROVED CERAMICCOMPOSITIONS FOR HIGH STABILITY CAPACITORS [75] Inventor: Rolland R.Roup, Alhambra, Calif.

[73] Assignee: Solid State Dielectrics, Inc.,

. Burbank, Calif.

[22] Filed: Jan. 26, 1971 211 Y App]. No.: 109,984

[56] References Cited UNITED STATES PATENT-S 2,908,579 10/1959- Nelsonetal 106/39 R 3,103,440 9/1963 Cline et al 106/39 R 2,432,250 12/1947Rath 106/39 R 2,520,376 8/1950 Roup et a1. 106/39 R 2,841,508 7/1958Roup et a1. 106/39 R 2,985,700 5/1961 Johnston.'. 106/39 R 3,364,1441/1968 Pulvari 252/629 OTHER PU BLlCATlONS Subbarao J Phys. Chem. Solids23 (1962) pp.

665-676 A Family of Ferroelectric Bismuth Compounds" [57] ABSTRACT Aceramic composition useful informing capacitors and having the followingcomposition expressed in mole percent of the metal oxides present:

neodymium oxide about 12 to about 20 mole percent; barium oxide about 12to about 20 mole percent; titanium oxide about 60 to about 70 molepercent; bismuth oxide about 1.5 to about 5 mole percent; zirconiumoxide O to about 5 mole percent; stannic oxide to about mole percent;calcium oxide and/or strontium oxide 0 to about mole percent rare earthoxides other than neodymium oxide i ranging up to about one mole percentbut preferably less than about 0.6 mole percent; titanium oxide stannicoxide zirconium oxide about to mole percent, and barium oxide calciumoxide strontium oxide about 12 to about 25 mole percent.

.coefficient of dielectric constant (TC) of about -l00 to about X 10'/C,a D.F. less than 0.1, and an insulation resistance corresponding tovalues of at least about 200 ohm-farads ranging up to about 40,000 ormore ohm-farads at C.

8 Claims, No Drawings IMPROVEDCERAM'IC COMPOSITIONS FOR i HIGH-STABILITYCAPACITORS This invention relates to improved ceramic dielectriccompositions for use in capacitors. More specifically,

the invention relates to improved ceramic dielectric compositions havinghigher values while, at the same time, having a temperature coefficientof dielectric constant which is relatively low, coupled with stability.to capacitance cha'nge'and high insulation resistance values.

In designing a capacitor having a given capacitance, the factorsgoverning the design are the'area of electri cally conducting electrodeplates on opposed sides of the ceramic dielectric elernent, thethickness of the ceramic dielectric, and the dielectric constant K ofthe ceramic dielectric or insulating material. If the capacitor is ofmultilayer design, thetotal area of the opposed electrode plates isconsidered. If all other factors are held constant, the capacitance isdirectly proportional to the'dielectric constant of the ceramicinsulatormaterial. Thus, if the dielectric constant of the ceramicdielectric is increased, the'area of the capacitor elements and theopposed electrode plates may be decreased in inverse proportion. v

If, however, the dielectric constant of the ceramic materialwasdecreased, it would benecessary to modify the other design parametersinorder toobtain a capacitor having the same capacitance value. Tocompensate for a decrease'in the dielectric constant ofthe dielectricmaterial, the area of the capacitor plates could be increased orthe'number of plates in the capacitor could be increased. Either ofthesedesign mod- I ifications would resultin increasing the cost of thecapacitor. This isespeciall-ytrue in the caseof multiple layer,monolithic dielectric capacitorswhichemploy.

v v 2 properties..When this is the case, the processing costs areincreased, due to the low yield of satisfactory parts. it is, therefore,desirable that the ceramic materials used in the capacitor may be firedsatisfactorily over a reasonably wide temperature range. f I

In accord with the present invention, l have providedceramic'compositions which produce dielectric materialshaving high Kvalues of about 55 to about 90 and higher. Unlike previous materials, myceramic componoble metal electrodes of the platinum-gold-palladiumgroup. As the number of electrode plates in such acapacitor isincreased, there is-a corresponding increase in the use of theexpensive'noble metal required for the electrodes. Thisresults inaconsiderable increase in the overall cost of the capacitor.

Another property of considerable importance in the design of thecapacitoris the temperature coefficient of dielectric constant '(TC) forthe dielectric material. The TC value is determined by the change-of thedielectric constantof the dielectric material with changes in thetemperature of the capacitor. Desirably,'tbe.tem-

perature coefficient of the dielectric constant for'many applicationsthe relatively low, i.e. 100 to +100 l0 /C. Even more desirably, thevalue ofthe temperature coetficient of dielectric constant is within theN.P.O. range which is from to +30 X 10"/C. The use of a dielectricmaterial which has a relatively low TC value provides a capacitor whosecapacitance is relatively constant over a temperature range. This isquite advantageous, for. example, if thecapacitor is a componentdesigned to maintain a uniform capacitance in an electrical system whichfunctions over a wide temperature range. I

Another factor which is of importance in the selection of a dielectricceramic material for use in a capacitor is the temperature range overwhich the dielectric materialcanbe fired to provide the desireddielectric properties. Many ceramics are quite sensitive inthis regardand must 'be' fired within a very narrow temperature range in order toobtain satisfactory dielectric sitions provide high K valueswhile, atthe same time,

providing temperature coefficients of dielectric constant within thelowTC range of about to about +100 10""/C. Preferably, the dielectricsprovidedby my ceramic compositions have TC values within the N.P.O.range of 30 to +30 X l0' /C. In addition, my compositionsprovide'ceramic dielectrics having a D.F. less than about 0.1 andpreferably less than about 0.03.

The insulation resistance (IR) of the ceramic dielectrics produced by mycompositions are such as to correspond to valves of at least-about 200ohmfarads at C and preferably 1000 ohm-farads or higher. I have obtainedinsulation resistance values for ceramic dielectrics produced by mycompositions which correspond to values in excess of 40,000 ohm-faradsat 125C. Moreover, my compositions may be fired over a reasonably widetemfined in terms of the various oxides which they contain. a Certain ofthese oxides may be present in a combined formin my compositions.However, this does not affect the overall definition of my compositionsin terms of ,the variousindividual metal oxides which they contain.

My compositions contain about 12 to about 20 mole percent of neodymiumoxide, about 12 to about 20 calcium oxide, strontium oxide, or a mixturethereof.

My compositions can also contain trace amounts of otheringredients'which are adventitiously present in ceramic compositions asimpurities. Rare earth oxides other than neodymium oxide may be' presentin trace amounts ranging up to about l-mole percent but are preferablypresent in amounts less than about 0.6 mole percent.

The total content of titanium oxide, stannic oxide and zirconium oxidepresent in my compositions ranges from about 60 to 70'mole'percent. Inthe. event that a particular composition contains, for example, stannicoxide, it may be necessary to reduce somewhat the titanium oxide contentor the zirconia content. The total content of barium oxide, calciumoxide and strontium oxide in my compositions ranges from about 12 toabout 25' mole percent. In the event that calcium oxide or strontiumoxide is present, the barium oxide content may, thus, 'be accordinglyreduced. f v 3 Several methods may be employed in formulating mycompositions. The initial step in their preparation involves wet millingthe various ingredients for several hours to produce fining and intimatemixing. This is conveniently accomplished in a ball mill with the finelydivided materials being discharged from the mill in the form of a slipmay be dried in any convenient manner,

such as by pan drying, belt drying, spray drying, etc. Following'drying,the material may be granulated, if desired, to break up any large clumpsof material. For example, if the material is pan dried, it may containlumps of material which may be conveniently broken up by forcing thematerial through a coarse screen, such as a ten-mesh screen. l lFollowing the granulating procedure, which is optional, the material isthen calcined at atemperature of about l000 to about l200C in anoxidizing atmosphere. After calcining, all the materials present in thecomposition are oxides. Using the above procedure for the formulation ofmy compositions, the various ingredients employed may be in a combinedstate of two or more oxides such asjbarium titanate, barium zirconate,barium stannate, bismuth zirconate, calcium zirconate and the titanatesof bismuth, calcium or strontium. The alkaline earth metals, e.g.,barium, calciumand strontium, are reactive in therein oxide forms withwater and carbon dioxide. Thus, they are used in my compositions in a'prereacted form as mixed metal oxides such as the titanates, zirconatesand stannates.

In the above procedure, all of the materials are calcined in anoxidizing atmosphere at a temperature of about l000 to about 1200C. asthe final step in their preparation. Inasmuch as the calcining isperformed in an oxidizing atmosphere, this permits the use of materialswhich are not oxides but which are converted to oxides during thecalcining operation. For example, the

neodymium oxide content of my compositions may be 52:48. The materialsmay be mixed to homogeneity by adding them to a ball mill along'withwater, as described previously, to form a slip. After being dischargedfrom the ball ,mill, the slip is then dried in any suitable manner. Iflumps are formed in the material during .the drying operation, thedrying may be followed by a granulation step as described previously.These materials are then calcined at a temperature of 'about l000 toabout 1200C. in an oxidizing atmosphere.

Following the calcining of the mixture of neodymium oxide and titaniumoxide, the calcined material may be added to a ball mill, together withthe other ingredients, and milled to homogeneity. Following this, thematerials may be dried'in any suitable manner and then granulated ifdesired. Following this,'the mixture of materials is then calcined at atemperature of about 1000 to 1200C in an oxidizing atmosphere. Thecalcining step is not necessary if all of the materials employed arealready in a pre-reac'ted state. The neodymium oxide may be in apre-reacted state due to calcining with titanium oxide as describedabove.' By way of example, barium titanate, barium zirconate, calcium.titanate, calcium stannate, bismuth titanate and bismuth zirconate arepre-reacted materials. Thus, if all of the materials employed are addedas titanates, stannates or zirconates, the final calcining step may beeliminated. Even when employing pre-reacted materials, it is still,however, preferable in some cases, to calcine the materials in the finalstep in the manner described above.

To-fuither illustrate myinvention, l have presented anumber of exampleswhich demonstrate my composi-.

tions and the properties which they provide when they are formed into aceramic dielectric material. In the examples, all parts and percentagesare by weight unless otherwise indicated.

vat llC in an'oxidizing atmosphere to form a prewere then formedintoceramic dielectric bodies.;This

was accomplished by milling the materials in a ball mill for about 20hours, together with an acrylic resin binder Acryloid B 7, Rohm & HaasGo). For each parts of ceramic powder, there were employed about 5 0parts of acrylic resin, an additional 50 to 100 parts of a chlorinatedhydrocarbon solvent, and about 0.4 to 0.5 parts'of plastercizer(sanitisizer After 1 milling for about 20 hours, the material was caston a glass plate to a dried thickness of about 3 mils with a doctorblade spreader. The cast material was air-dried at room temperature andwas then stripped from the plate. Following this,.the strip was cut intorectangles measuring approximately 3 inches X 1 inch, and a plurality ofthe rectangular piece swere then compressed under about 10,000 lbs. ofpressure for a short period such as 10 to 20 secondsto provide a bodyhaving a compressed thickness of about 0.026 to about 0.028

inches. This material was then diced into one-half inch squares and thesquareswere fired in a continuous electric tunnel kiln while resting onzirconium oxide plates. The kiln was operated on a 7-hour cycle, i. e.,about 7 hours to pass through the kiln with the material being 1maintained for about 1 .hour at the elevated firing temperatures shownin the following table.

Following the firing step to form a ceramicdielectric body, the oppositesides of each: of the bodies, which had been reduced in size by about 25due to shrinkmeasurements were taken to determine the dielectricproperties of the ceramic material. The various dielectric propertiesfor the ceramic material which were measured are set forth in the table.

The above described-procedure for forming a ceramic dielectric isconventional and does not form a produce dielectric materials having Kvalues in the order of about 50 to 90 while, at the-same time, havingrelatively low TC values. The higher K values are achieved withoutsacrificing the stability of the dielecpart of my 1nvent1on. Any of thefabrication methods 5 tr1c constant w1th respect to temperature changes.The known to the art may be used in forming my composi- %D.F. values, asshown on the table, further demontions into ceramic dielectrlcs. stratethat the dielectric materials of the present inven- TABLE I (WeightPercent) NdzOa to T102 Firing Example 21 temp., Percent Ohmnumber T102Ndzoa C. K D.E. ,T.C tarad With reference to the above table, theExample numbers are set forth in the first column and the concentrationsof the various materials are set forth in weight percent in ColumnsZ-ll.In Column 4, the material indicated as bismuth titanate is a mixture ofabout 72-74 percent by-weightof bismuth oxide and 25-27 percent byweight of titanium dioxide; In Column 5, the material indicated asbismuth zirconate is a mixture of approximately 72% by weight of bismuthoxide and 27% by weight of zirconium dioxide with about 1.0 weightpercent of silicon dioxide.

In Column 9 is indicated the concentration by weight of pre-reactedneodymium oxide with titanium dioxide at a weight ratio of neodymiumoxide to titanium dioxide of 58:42. lnCol'umn is indicated the weightpercent of titanium dioxide and in Column 1 l is the weight percentconcentration of neodymium oxidefln Examples 1 7, where the neodymiumoxide and titanium dioxide werepre-reacte d before being admixed withthe other ingredients'in the composition, the percentages shown inColumns 10 and l 1 indicate, respectively, the total amount of titaniumdioxide and neodymium oxide which are present in a pre-reacted state inthe composition. In Examples 8-13, the titanium dioxide and thedetermined at 125C. While at l25C,500 volts was neodymium oxide were notemployed in a pre-rea'cted state. v

Column 12 in theabove-table indicates the firing temperature that wasemployed in forming the composition into a ceramic dielectric body. Thematerials were maintained at the indicated firing temperatures for about1 hour while being passed through the continuous tunnel kiln in themanner described previously.

The K values ordielectric constants for the ceramic bodies are set forthin Column 13. Thesevalues were measured at a temperature of 25C. The%D.F. values shownin Column l4'were also measured at 25C and one KC. TheTC values set forth in Column 15 are expressed in parts per million perdegree C or 10 /C.-

These values were measured over the temperature range from 25.to 125C.The ohm-farad values set forth in Column 16 were measured at 125C.

impressed across the ceramic dielectric for 2 minutes and theyresistance of the ceramic dielectric was then immediately read. T hecapacitance of the ceramic dielectric was also measured at. 1259C. Theohm-farad values were obtained by multiplyingthe'measured resistance inohms at 125C by the measured capacitance in farads at thesametemperature.

' The significance of the ohm-farad value is that it is a constant forthe dielectric material and is a basis for comparing capacitors ofdifferent size. The high ohmfarad values demonstrate that the electricalleakage of my dielectrics is low'such that they may be used in ca-'pacitors having very high capacitance values.

The various ingredients employed in the Examples of Table l arecommercially available-from TAM Division, National Lead Corporationorfrom Transelco Corporation. The barium titanate contained 33.5 to 33.9per-. cent by weight of titanium dioxide and 63.8 to 64.2 weight'percentof barium oxide together with minor amounts of silicon dioxide, aluminumoxide, strontium oxide and sodium oxide. The barium zirconate contained41.0 to approximately 44.0 weight percent of zirconium dioxide, 53.0 to55.0 weight percent of barium oxide, about 2.0to 3.0'weight percent ofsilicon dioxide and minor amounts of titanium dioxide and aluminumoxide. The calcium stannate contained 64.80

percent by weight of stannic oxide and 23.91 percent by weight ofcalcium oxide-while the calcium titanate contained 39.0 to 41.0 weightpercent of calcium oxide and 55.0 to about 59.0 weight percent oftitanium dioxide-with minor amounts of 'silicon dioxideand-alumicomposition, as shown in Table I, are set forth in weight TABLEII (Mole Percent) T102 I plus BaO S110: Example plus plus number BaO0210 B1 Nd O; T102 S1102 Zl'Oz CaO ZrOz As illustrated by Table 11,above, my compositions having metal oxide contents within the percentageranges previously indicated provide ceramic dielectrics havingsuperiorproperties. When Example 11 is repeated with a substitution of strontiumtitanate for cal- I cium titanate a dielectric having good electricalproperties is obtained.

An additional advantage in the use of my compositions is that theyproduce essentially non-porous dielectric materials. The porosity of theresulting ceramic dielectrics may be-conveniently determined accordingto an ink stain test. In this test, the ceramic dielectricmaterial,'which has been fired in the manner described previously, isimmersed in ink for about 30 seconds.

Following this, the dielectric material is removed from the ink, rinsedquickly in water and its surfaces wiped off with a dry cloth. If thematerial is essentially nonporous, there will be no visible ink stain onthe surface and the material is given a test rating of 1. If there is alight surface stain, the material is given a 2 rating. If there is somepenetration of the ink into the body of themateriaL in addition to thesurface staining, the material is given -a 3 rating and if there is agreat deal of penetration into the body, of the material, the materialis given a 4 rating. The dielectric materials produced by mycompositions gave fairly consistent ratings of 1 according to the abovetest, thus demonstrating the essentially non-porous nature of theresulting ceramic dielectrics. The ink used in the testis not criticaland any water base inkor dye may be employed.

I In forming the compositions of my invention, the

concentrations of the various ingredients may be adjusted so as to varythe TC value of the resulting dielectric material. Neodymium oxide andbismuth oxide are positive TC shifting ingredients; calcium, strontium,zirconium oxide and stannic oxide are negative TC shifting, and titaniumis a positive shifter. Bismuth titanate and barium titanate are fairlyneutral in regardto shifting the TC of the dielectric. By increasing thenegative shifting TC ingredients, the TC of the dielectric can be mademore negative. Conversely, an increase in the positive shifting ,TCingredients results in a more positive TC value for the dielectric.

Various materials, such as mineralizers or electrical modifiers may bepresent in my compositions in minor A sition consisting essentially ofabout 12 to about 20 mole percent neodymium oxide, about 12 to about 20mole percent barium oxide, about 60 to about mole percent of titaniumoxideyabout 1.5 to about 5 mole percent of bismuth oxide, from 0 toabout 5 mole percent of zironium oxide, from O to about 5 mole percentof stannic oxide, from '0 to about '10 mole percent of calcium oxide,strontium oxide 'or mixtures thereof, and having a content of rare earthoxides other than neodymium oxide ranging up to about one mole percent,the total content'of said titanium oxide, stannic oxide and zirconiumoxide ranging from about 60 to about 70 molepercent and the totalcontent of barium oxide, calcium oxide and strontium oxide ranging fromabout 12 to about 25 mole percent. v 2. The composition of claim 1wherein the barium oxide, calcium oxide and strontium oxide are presentas titanates, zirconates and stannates.

3. The composition of claim 2 wherein the bismuth oxide is present as atitanate, zirconate or stannate.

4. The composition of claim'l wherein'said neodymium oxide is present ina pre-reacted combined form with titanium oxide at a weight ratio ofneodymium oxide to titanium oxide of about 68:32 to about 52:48. Thecomposition of claim 3 wherein said neodymium oxide is present in a,pre-reacted combined form with titanium oxide at a weight ratio ofneodymium oxideto titanium oxide of about 68:32 to about 52:48.

6. A dense, substantially non-porous ceramic dielecme material having ax value of about 55 to about 90, a temperature coefficient of dielectricconstant of ide, about 1.5 to about 5 mole percent of bismuth ox ide,from 0 to about 5 mole percent of zirconium oxide, from 0 to about 5mole percent'of stannic oxide, from 0 to about 10 mole percent ofcalcium oxide, strontium oxide or mixtures thereof, and having acon-tent of rare earth oxides other than neodymium oxide ranging toabout one mole percent, the t'otal content of said titanium oxide,stannic oxide and zirconium oxide ranging from about 60 to about 70 molepercent and the total content of barium oxide, calcium oxide andstrontium oxide ranging from about '12 to about 25 mole percent. 7. Theceramic dielectric of claim 6 having a %D.F. of 0.03 or less.

8. The ceramic dielectric of claim 6 having an insulation resistancecorresponding to values of at least about 1000 ohm-farads.

NIT D PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3J75,142. vDatedNovember 27, 1973 Inventor(s) Rolland R. Roup It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column l, line 50, "the" should read is Column 2, 21, valves" shouldread values Column 4,'line 4l,. lascercizer" should read f--plasticizer.-

I'able I, s should read D.F.

{IableI, column T.C., "+27" should read -27 lla im 6, column 8, line 58,"ranging to" should read ranging up to Signed: and sealed this 26th dayof November 1974.

EAL) Atte st: McCOY M.. GIBSON JR. c. MARSHALL DANN Atte' st ingOrifice: Commissioner of Patents FORM (1M9) uscoMM-oc 60376-P69 U. 5.GOVERNMENT PRINTING OFFICE 2 "9 0-356-33L

2. The composition of claim 1 wherein the barium oxide, calcium oxideand strontium oxide are present as titanates, zirconates and stannates.3. The composition of claim 2 wherein the bismuth oxide is present as atitanate, zirconate or stannate.
 4. The composition of claim 1 whereinsaid neodymium oxide is present in a pre-reacted combined form withtitanium oxide at a weight ratio of neodymium oxide to titanium oxide ofabout 68:32 to about 52:48.
 5. The composition of claim 3 wherein saidneodymium oxide is present in a pre-reacted combined form with titaniumoxide at a weight ratio of neodymium oxide to titanium oxide of about68:32 to about 52:48.
 6. A dense, substantially non-porous ceramicdielectric material having a K value of about 55 to about 90, atemperature coefficient of dielectric constant of about -100 to +100 X10 6/*C, a %D.F. less than 0.1, an insulation resistance such as tocorrespond to values of at least about 200 ohm-farads at 125*C, saiddielectric material having a composition consisting essentially of about12 to about 20 mole percent neodymium oxide, about 12 to about 20 molepercent barium oxide, about 60 to about 70 mole percent of titaniumoxide, about 1.5 to about 5 mole percent of bismuth oxide, from 0 toabout 5 mole percent of zirconium oxide, from 0 to about 5 mole percentof stannic oxide, from 0 to about 10 mole percent of calcium oxide,strontium oxide or mixtures thereof, and having a con-tent of rare earthoxides other than neodymium oxide ranging to about one mole percent, thetotal content of said titanium oxide, stannic oxide and zirconium oxideranging from about 60 to about 70 mole percent and the total content ofbarium oxide, calcium oxide and strontium oxide ranging from about 12 toabout 25 mole percent.
 7. The ceramic dielectric of claim 6 having a%D.F. of 0.03 or less.
 8. The ceramic dielectric of claim 6 having aninsulation resistance corresponding to values of at least about 1000ohm-farads.